In prior art two-piece golf balls, the core is typically formed from rubber materials based on highly resilient polybutadiene rubber and heat crosslinked with a peroxide and a co-crosslinking agent such as methacrylic acid. The cover is commonly formed of ionomer resins featuring impact resistance and cut resistance, commercially available under the trade name of Surlyn and Himilan.
The two-piece golf balls are generally given the advantages of distance and durability at the sacrifice of a soft feel, which is very important to golf players. Efforts were made to develop two-piece golf balls with a soft feel by making the core softer and the cover harder. These golf balls, however, suffer from other problems. Some golf balls are less durable against consecutive strikes, some lack rebound characteristics, and some have poor low-temperature performance.
As a result of the ensuing development work, there were marketed golf balls whose core or cover has a two layer structure in which resilience is assigned to one layer and a feel is assigned to the other layer.
The ionomer resins favored as the cover material and the crosslinked polybutadiene favored as the core material manifest good resilience at a high hardness, but tend to lose resilience as the hardness lowers. While polyurethane elastomers, polyamide elastomers and polyester elastomers are advantageously used as the cover intermediate layer for imparting a comfortable feel, they are desired to have further resilience and the reduced dependency of hardness on temperature.
An attempt was then made to blend millable type silicone rubber in polybutadiene, followed by peroxide crosslinking (JP-A 60-258236). Problems arise with respect to the dispersion of silicone rubber in polybutadiene. Non-uniform reactivity leads to low durability against strikes. This combination is then very difficult to consistently provide good physical properties.
Another attempt was made to blend millable type silicone rubber in a core-forming rubber or cover-forming resin, followed by dynamic crosslinking (JP-A 8-243191 and JP-A 9-220296). These blends have drawbacks including extremely reduced flow, difficulty of injection molding, and difficulty of uniform dispersion, which lead to low durability against strikes. It is very difficult to tailor these blends to a practically acceptable level.
The development of a new material is indispensable to impart desirable characteristics to the golf ball. In the golf ball industry, there is a need for a material having further increased resilience to enable better flight performance, softness, and stable physical properties over a wide temperature range.